This invention relates generally to laser therapy and, in particular, methods and apparatus for conditioning a laser beam for distribution to a therapy site.
Laser radiation emitted from the confines of a laser cavity is typically non-uniform in intensity. Most commonly, lasers exhibit Gaussian intensity profiles across at least one axis in the so-called TEM.sub.oo mode of operation. Many profiles are possible, however, and depend upon a variety of factors, including the laser cavity geometry and alignment.
Non-uniformity in a laser beam profile often can pose problems, particularly in laser therapy applications. Irradiation of human tissue during surgery or therapy usually requires precise control of exposure time and power. When a therapeutic laser is coupled with a multi-fiber surgical tool, e.g., a catheter carrying a plurality of optical fibers, the problem of non-uniformity can become pronounced because the inhomogeniety in the laser beam's spatial intensity is transferred directly to the fibers within the tool. In regions of lower intensity, e.g., toward the edges of the therapeutic laser beam, the receiving fiber typically will transmit radiation of a lower intensity. Near the optical axis of the laser beam, the receiving fiber will typically transmit radiation of a higher intensity.
Consequently, the laser power delivered at the distal end of a multi-fiber surgical tool and equivalently at the target tissue region, can be very difficult to control. When a plurality of laser fibers are used to deliver radiation to a target region, the intensity of radiation delivered by each fiber can vary widely.
Methods do exist to homogenize the intensity profiles of lasers, but they are often inefficient. For example, high energy lasers are sometimes put through a process known as "beam-scraping." This process approximates a spatially-uniform, laser beam intensity by allowing the beam to expand and then blocking off (e.g., with an aperture or diaphragm) regions of lower intensity. After one or more passes through a beam-scraping system, the laser beam is clipped to provide a top hat-like profile.
However, this is wasteful since a substantial amount of energy is dissipated during beam scraping operations. It is also generally not desirable for systems in which therapeutic laser radiation must be transmitted via optical fibers. In such cases, intensity control is desired without significant energy loss.
It is, accordingly, an object of this invention to provide apparatus and methods for uniformly transferring laser radiation from a source to a plurality of optical fibers without substantial loses in energy.
It is another object of this invention to provide apparatus and methods for delivering laser radiation to surgical tools.
Yet another object of this invention is to provide apparatus and methods to enable greater precision and control in laser surgical instruments and applications.
These and other objects of the invention are evident in the description that follows.